The use of springs, particularly tension springs, to provide a counterbalance force for the vertically moveable sash to double hung windows has long been known. Various techniques have been developed to counterbalance the force generated by the spring with the weight of the windows, such that the sash will remain stationary in any vertical position of the sash if the window is released by the operator in that particular position. This has been accomplished in a number of ways, as disclosed for example in prior U.S. Pat. Nos. 3,788,006, 4,015,367 and 4,570,382, 4,571,887, 4,763,447 and 4,779,380. However, none of these patents have developed a truly simple structure which is capable of being used with sash of a widely-varying range of sizes and weights without impairing the counterbalancing effectiveness of the mechanism such that either the sash is overly difficult to move in at least certain of its possible positions or else it will not reliably remain in position when moved to a position in which the spring is stretched beyond a given point.
In attempting to develop this balance of forces, it is very important that the balance mechanism be kept simple and its cost minimized. If these requirements are not satisfied, the result is a non-competitive product which will not find commercial acceptance. It is also important that the balance system, including the jamb liner and balance, be easy to install and durable in use. This latter is very important when the system is installed in commercial buildings, such as apartments and the like, where use may be frequent and the lack of durability and dependability will result in excessive maintenance problems.